1. Field of the Invention
The present invention relates generally to a fluid pump unit serving as a power source of an automotive power steering system or the like. More specifically, the invention relates to a fluid pump unit with a flow control valve which returns excessive discharge to the interior of the fluid pump unit.
2. Description of the Background Art
A fluid pump unit is widely used as a power source of an automotive power steering system. Generally, amount of working fluid discharged from such a fluid pump unit per unit time increases in proportion to increase of the engine speed, since discharge amount is essentially constant per one cycle thereof. On the other hand, in order that automotive power steering systems stably exhibit their performance, required and sufficient working fluid must be supplied thereto. In addition, the required maximum discharge amount of working fluid is determined by the maximum rotation speed of a steering wheel. Therefore, when discharge amount of working fluid exceeds the required maximum value due to increase of the engine speed, the fluid pump unit must be provided with a system, such as a flow control valve, which returns excessive discharge to the interior thereof. This flow control valve causes the amount of working fluid supplied to the power steering system to decrease as the engine speed increases and while the engine speed is relatively high, so that the vehicle will handle stably at high speed.
Flow control valves of this type generally comprise a spool chamber communicated with discharge side of a fluid pump unit, and a valve spool movably housed within the spool chamber. The valve spool moves in response to discharge pressure of the fluid pump unit. Movement of the valve spool is designed to adjust open an area of a drain passage which establishes fluid communication between the discharge side and an inlet side thereof. In this way, when the amount of working fluid discharged from the fluid pump unit becomes greater than a predetermined value, a part of the discharged working fluid returns to the inlet side of the fluid pump unit, so that the amount of working fluid supplied to an external load, such as an automotive power steering system, can be adjusted.
However, in conventional fluid pump units, the spool chamber has only one drain port for the drain passage, and the drain passage branches to two passages on the way to be communicated with the inlet side thereof. Therefore, there is a disadvantage in that abrasion occurs partially on the valve spool and the spool chamber, since the valve spool slides within the spool chamber while it is thrust against the inner wall thereof on the side of the drain port. In addition, there is a disadvantage in that noise occurs due to agitation and cavitation of working fluid at the branching portion of the drain passage.